Several methods can be used to modify the level of superoxide dismutase in bacteria. Among these are: growth in the presence of different levels of oxygen: growth in the presence of substances which increases the rate of intracellular production of O2 at a fixed concentration of oxygen: selection of mutants with defects in superoxide dismutase and selection of mutants that are derepressed in the synthesis of these enzymes. The level of superoxide dismutase, modified by any of these methods, can then be correlated with resistance towards the lethality and towards the mutagenicity of oxygen. The functions of the periplasmic iron-containing superoxide dismutase and of the matrix manganese-containing enzyme in gram bacteria will be explored by creating mutants with specific defects in one or the other of these enzymes. O2 has been shown to be important in the oxygen-dependent killing of bacteria by human neutrophils. The enzyme which makes this O2 is a membrane-associated NADPH oxidase. This activity is also present in placentae and the responsible enzyme is being isolated. Once in hand, it will be compared to that in the neutrophils by immunological methods. BIBLIOGRAPHIC REFERENCES: A Convenient Calibration of the Clark Oxygen Electrode. Misra, H. P., and Fridovich, I., 1976. Anal. Biochem. 70, 632-634. The Accumulation of O2 During the Aerobic Action of Xanthine Oxidase: A Requiem for H2O4. Hodgson, E. K., and Fridovich, I., 1976. Biochim. Biophys. Acta 430, 182-188.